Preheat the oven to 350°. Spread the challah on a baking sheet and toast until dry, about 8 minutes. Let cool, then pulse the bread into coarse crumbs in a food processor. Spread one-third of the crumbs in a buttered 9-by-13-inch baking dish in an even layer.
In a small bowl, combine the sugar with the nutmeg, cinnamon, lemon zest, orange zest and salt. Set aside.
Spread half of the apples over the crumbs in the baking dish and top with half of the sugar mixture. In another bowl, mix the cider with the lemon and orange juices; drizzle over the apples. Cover with another third of the crumbs and the remaining apples and sugar mixture. Cover with the remaining crumbs. Using a sharp knife, shave the margarine as thinly as possible; dot it all over the top.
Bake the apple Betty in the center of the oven for about 1 hour, until the apples are tender and the juices are bubbling; cover the baking dish during the last 15 minutes to prevent the crumbs from getting too dark. Let cool for 30 minutes, then serve with pareve vanilla ice cream or pareve whipped cream.